1. Field of the Invention
The present invention relates to linear beam amplification devices having an electron emitting cathode and an RF modulated grid closely spaced therefrom, and more particularly, to a novel support structure for the grid that accommodates thermal expansion while maintaining an optimum grid-to-cathode spacing.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron or travelling wave tube amplifier, to generate or amplify a high frequency RF signal. Such devices generally include an electron emitting cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture. One class of linear beam device, referred to as an inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode. As the density modulated beam propagates across a gap provided downstream within the IOT, RF fields are induced into a cavity coupled to the gap. The RF fields may then be extracted from the cavity in the form of a high power, modulated RF signal. An example of an IOT is provided by U.S. Pat. No. 5,650,751, to R. S. Symons, for INDUCTIVE OUTPUT TUBE WITH MULTISTAGE DEPRESSED COLLECTOR ELECTRODES, the subject matter of which is incorporated in the entirety by reference herein.
Since it is desirable to space the grid closely to the cathode surface, it thus must be capable of withstanding very high operating temperatures. In view of these demanding operating conditions, it is known to use pyrolytic graphite material for the grid due to its high dimensional stability and heat resistance. The pyrolytic graphite grid may be made very thin, with a pattern of openings formed therein, such as by conventional laser trimming techniques, to permit passage of the electron beam therethrough. The low coefficient of expansion of the pyrolytic graphite permits the grid to be heated by direct thermal radiation from the cathode and by dissipation of RF drive power when applied between the cathode and grid, without expanding the grid into the cathode and shorting these two elements together. As a result, the grid may be positioned very close to the cathode surface, permitting low RF drive voltage and high gain.
Nevertheless, a practical limitation on the efficiency of such linear beam devices has been the difficulty of supporting the grid in a proper position relative to the cathode surface. A metallic grid support structure, such as comprised of copper or stainless steel, will thermally expand at a much greater rate than the pyrolytic graphite grid, causing the relatively delicate grid to rupture. It is known to dispose a resilient, annular contact element between the grid and the metal grid support structure, such as a metallic braid, that compresses upon thermal expansion of the grid support structure to maintain proper spacing between the grid and the cathode surface. A drawback of this construction is that the resiliency of the contact element tends to degrade over time, especially due to the repeated thermal cycling of the device. Moreover, the addition of the contact element increases the complexity and associated cost of manufacture of the device.
Thus, it would be very desirable to provide a grid support structure for a linear beam device that maintains a proper spacing between the cathode and grid across the operating temperature range of the device. It would be further desirable to provide such a grid support structure which avoids the use of an additional resilient member.